1. Field of the Invention
The present invention relates to a method for treating gases having a predetermined flow direction in a gas conduit comprising the steps                a) injecting a powdery compound into the flue gas conduit at an injection point of a powdery compound, via a powdery compound injection piping, said injection piping having an external face and an internal face arranged so as to be in contact with said powdery compound, so as to allow formation of a cloud or flow of particles of powdery compound in said gas conduit,        b) injecting a monophasic liquid aqueous phase, as droplets, into said gas conduit,        c) capturing pollutants of the gases with said powdery compound, and        d) separately recovering said powdery compound enriched in pollutants and the gases depleted in pollutants.        
2. Description of the Prior Art
Typically, the treatment of gases, in particular flue gases, requires reduction of acid gases, notably HCl, SO2 and/or HF, which reduction may be carried out under dry conditions, by injecting a substance, often mineral, dry and powdery into a flue gas flow or through a filter-bed comprising solid particles either fixed or in motion. In this case, the powdery compound generally comprises a calcium-magnesium compound, in particular lime, preferably slaked or hydrated lime or a sodium compound like a sodium carbonate or bicarbonate. Other mineral compounds may also be used notably those used for reducing dioxins, furans and/or heavy metals including mercury, like for example, those based on phyllosilicates, such as seplolite or halloysite or the like.
The present invention is more particularly directed to methods for reducing acid gases in gases, in particular flue gases, by injecting a powdery product having capabilities of capturing acid pollutants in the gas flow in order to improve the reducing yields of the acid gas components of the treated gases.
More particularly a treatment of these flue gases with powdery hydrated lime is known, improved by the use of humidified hydrated lime instead of dry powdery hydrated lime. Indeed, the absorption of the pollutants of the gas phase by the particles of the solid phase is improved by the presence of water. Sometimes such a flue gases treatment is described as a semi-dry method. A method of using hydrated lime and water is also known wherein milk of lime is injected into the flue gases flow. Unfortunately, the injection of milk of lime assumes the implementation of specific means (dispersion turbines, circulation pump) which are energy consuming and subject to wear and to erosion and may lead to clogging problems. Documents CN 2011 68568 and JP 10-216 572 each propose an alternative intended to solve the clogging problems by making the milk of lime suspension in situ during the injection. For this purpose, a lance consisting of two concentric tubes, i.e. an internal tube and an external tube, is used. The water is injected via the internal tube while the hydrated lime is injected via the external tube. According to CN 2011 68568, the position of the internal tube may be adjusted relatively to the position of the external tube and it is therefore possible to have an internal tube for the water having a protruding or retracted output orifice or further at the same level with respect to an output orifice of hydrated lime of the external tube.
According to these documents, the design of the spraying lances imposes that the water projection tube be internal to the tube for projection of powdery hydrated lime. Therefore the result of this is that the sprayed lime encounters an obstacle within the external spraying tube which lies in the Internal tube for projecting water. In this way, the hydrated lime comes into contact with the generally metallic and cold surface of the internal tube, on which the lime tends to be clogged.
Further, according to these documents, the humidity level of the hydrated lime induces a water/lime mass ratio from 5 to 6 in order to form a suspension in situ. This suspension actually provides an ancillary detrimental effect for the treatment of flue gases in that the gas is further significantly cooled upon injection of hydrated lime. This assumes the availability of sufficiently hot flue gases in order for depollution to be effective; however this is not sought today while industrialists have an increasingly pronounced tendency of recovering at best the heat of the flue gases for economical and environmental reasons.
Moreover, when the hydrated lime comes into contact with a cold surface, as this is the case of the surface of the internal tube for projecting water of these documents, condensation occurs within the hydrated lime projection tube, which also has the result of promoting clogging of the projection tube and thus perturbing the injection of lime into the flue gases flow.
Finally, a water/lime mass ratio as high as the aforementioned one assumes availability and large consumption of water.
A device for injecting lime into flue gases is also known, wherein the powdery or hydrated lime injection piping is surrounded by an external concentric piping through which gas is injected (cf. US 2012/0251423).